(a) Field of the Invention
The present invention relates to an automatic transmission for vehicles. More particularly, the present invention relates to a shift control method for an automatic transmission which temporarily controls the automatic transmission into a neutral state when the vehicle is stopped in a first speed of a drive D range, thereby reducing drone noise and minimizing fuel consumption.
(b) Description of the Related Art
In the automatic transmission used for vehicles, a shift control system performs control to realize automatic shifting into different speeds and shift ranges according to various factors including throttle opening, vehicle speed and load, and several other engine and driving conditions detected through a plurality of sensors. That is, based on such various factors, the shift control system controls a plurality of solenoid valves of a hydraulic control system such that hydraulic flow in the hydraulic control system is specifically directed, ultimately resulting in shifting of the transmission into the various speeds and shift ranges.
In more detail, when the driver manipulates a shift lever to a particular shift range, a manual valve of the hydraulic control system undergoes port conversion as a result of the manual valve being indexed with the shift lever. Accordingly, hydraulic pressure supplied from a hydraulic pump selectively engages a plurality of friction elements of a gear shifting mechanism according to the duty control of the solenoid valves, thereby realizing shifting into the desired shift range.
Through such operations, as a result of a specific transfer of hydraulic pressure in the hydraulic control system, the friction elements of a low-reverse brake, an underdrive clutch and a reduction brake are selectively engaged to realize shifting into a first speed of a drive D range; the friction elements of a second brake, the underdrive clutch and the reduction brake are selectively engaged to realize shifting into a second speed of the drive D range; the friction elements of the underdrive clutch, an overdrive clutch and the reduction brake are selectively engaged to realize shifting into a third speed of the drive D range; the friction elements of the underdrive clutch, the overdrive clutch and a direct clutch are selectively engaged to realize shifting into a fourth speed of the drive D range; the friction elements of the second brake, the overdrive clutch and the direct clutch are selectively engaged to realize shifting into a fifth speed of the drive D range; the friction elements of the low-reverse brake, the reduction brake and a reverse brake are selectively engaged to realize shifting into a reverse R range; and the friction elements of the low-reverse brake and the reduction brake are selectively engaged to realize shifting into a neutral N range and a park P range.
In such an automatic transmission, shift quality is determined by how smoothly the friction elements are engaged and disengaged. Namely, when changing shift ranges and especially when changing speeds in the drive D range, the timing between the engagement of a specific set of friction elements in relation to the disengagement of another specific set of friction elements determines the shift quality of the automatic transmission. Accordingly, there have been ongoing efforts to develop improved shift control methods that enhance shift quality by better controlling the timing of friction elements to engaged and disengaged states.
With the use of conventional shift control methods, when the vehicle is temporarily stopped in the drive D range by the driver depressing the brake pedal, the first speed of the drive D range is maintained through the selective engagement of the low-reverse brake, underdrive clutch and reduction brake. If the vehicle is stopped for an extended period, for example at a stop light, substantial drone noise is generated both as a result of slippage between the engaged friction elements and by a turbine of a torque converter. Further, a considerable amount of fuel is consumed in such a state since engine load is maintained while in the first speed of the drive D range, resulting also in the generation of significant exhaust emissions.